The present invention relates generally to an improved fuel rod design for use in nuclear reactors. More paticularly, the present invention provides nuclear reactor fuel rods in which one or more metal oxides are present within the fuel rod to mitigate secondary hydriding.
When an LWR fuel rod cladding is breached, for example as a result of debris fretting, the coolant water/steam ingresses to the interior of the fuel rod where an oxidation reaction occurs with the fuel and zirconium alloy cladding to produce hydrogen and zirconium and uranium oxide moieties. The net effect of this oxidation reaction is that the oxygen in the steam is progressively removed and the interior space of the fuel rod is filled with a mixture of hydrogen and steam. At a sufficient distance from the primary breach location, the hydrogen is extremely dry, as most of the steam has been reacted out. Under these conditions, the hydrogen is rapidly absorbed by the cladding to form massive secondary hydrides which are brittle in nature. Subsequent loading of the cladding leads to a new rupture at these secondary hydrided locations. The rupture may be cirucumferential or, in some instances, could lead to axial crack propagation. In all cases, there is additional exposure of the fuel and fission products to the coolant. For this reason, it is important to mitigate secondary hydriding of the cladding.
The conditions that relate to the formation of secondary hydrides in zirconium-alloy cladding have been discussed extensively in the literature. It is now well-recognized that massive secondary hydriding takes place when the steam fraction in the steam-hydrogen mixture interior to the cladding falls below a threshold level. Very dry hydrogen conditions are generally needed for massive secondary hydriding of the cladding, and even small quantities of steam will serve to mitigate secondary hydriding.
A need exists for a fuel rod design which will not be susceptible to secondary hydriding of zirconium-alloy fuel cladding in the event of cladding breach and ingress of water or steam to the interior of the fuel rod. The present invention seeks to fill that need.
It has now been discovered, surprisingly, that secondary hydriding can be mitigated or eliminated by providing one or more metal oxides within the fuel rod. The invention is particularly directed to providing improved fuel rod design for use in a Light Water Reactor (LWR).
In a first aspect, the invention provides a method of fabricating a fuel rod in which the tendency for secondary hydriding is mitigated, comprising the step of providing an effective amount of a metal oxide in the fuel rod. The composition of the metal oxide is generally such that if the hydrogen fraction is above the equilibrium condition for the M/MOx couple, a back reaction occurs between the hydrogen and the metal oxide to generate steam and mitigate secondary hydriding. The metal oxide may be selected from oxides of iron, nickel, tin, bismuth, copper, colbalt, chromium, manganese and/or combinations of such oxides.
In a further aspect, the invention provides a fuel rod with reduced tendency to undergo secondary hydriding, fabricated according to the method of the invention.